Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices include a plurality of optical element modules including optical elements, such as lenses, mirrors, gratings etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to illuminate a pattern formed on a mask, reticle or the like and to transfer an image of this pattern onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups that may be held within distinct optical element units. Facet mirror devices as the ones mentioned above, among others may serve to homogenize the exposure light beam, i.e. to effect a power distribution within the exposure light beam which is as uniform as possible.
Due to the ongoing miniaturization of semiconductor devices there is not only a permanent desire for enhanced resolution but also a desire for enhanced accuracy of the optical systems used for fabricating those semiconductor devices. This accuracy obviously not only has to be present initially but has to be maintained over the entire operation of the optical system. A particular problem in this context is proper heat removal from the optical components to avoid uneven thermal expansion of these components leading to uneven deformation of these components and, ultimately, to undesired imaging errors.
As a consequence highly sophisticated facet mirror devices have been developed such as disclosed, for example, in DE 102 05 425 A1 (Holderer et al.) and DE 103 24 796 A1 (Roβ-Meβemer), the respective entire disclosure of which is incorporated herein by reference.
Both these documents, among others, show facet mirror devices where facet elements with a spherical rear surface sit in an associated recess within a support element. The spherical rear surface rests against a corresponding spherical wall of the support element confining this recess. While such a sphere to sphere interface theoretically may provide a large area of contact with good heat transfer from the facet element to the support element, this large area contact mainly depends on the manufacturing accuracy of both the facet element and the support element. In particular, the spherical recess can be rather expensive to manufacture at an accuracy of a few microns or less, as is desirable in many cases in all three directions in space.
To overcome this problem DE 103 24 796 A1 (Roβ-Meβemer) suggests to place a relatively soft coating (e.g. a gold coating) onto one of the spherical surfaces which compensates manufacturing tolerances by deformation. However, despite the low rigidity of this coating, due to the large contact area such deformation can involve relatively large forces which can introduce undesired deformation into the facet element.
Another approach is disclosed in DE 102 05 425 A1 (Holderer et al.) wherein the spherical rear surface of the facet element, more or less in a line contact, rests against a conical wall confining the recess receiving the facet element. This solution, due to the line contact can provide a lower heat transfer while still not considerably reducing the manufacturing effort for the conical wall to have the desired accuracy.
A third approach to support the facet elements is disclosed in DE 102 05 425 A1 (Holderer et al.) wherein the spherical rear surface of the facet element, more or less in a three point contact, rests against three small spheres each located at a free end of a support pin element. Here, the heat transfer can be even worse without considerably reducing the manufacturing effort for the three small spheres to have the desired accuracy.
In all three cases outlined above, a manipulating lever is connected to the rear surface of the facet element, and corresponding manipulators act on the manipulating lever to adjust the position and, predominantly, the orientation of the facet element with respect to the support element. Furthermore, in some cases, the manipulating lever is used for fixing the facet element relative to the support element once it has been adjusted. This solution can have the disadvantage that the manipulating lever adds to the complexity and, ultimately, to the cost not only of the facet element but also of other components such as the support element. Furthermore, multiple manipulators are used to generate mutually counteracting manipulation forces to allow accurate adjustment in a reasonable amount of time.